1. Technical Field
This invention relates to digital computer graphics processing and is especially applicable for use in the image generators of visual simulation systems.
2. Discussion
The process of creating digital computer images for display in a visual simulation system from visual databases describing the mathematical representations, or geometric models, of objects, called pixelizing, inherently produces an undesirable effect on the digital computer image that is ultimately displayed for view. Known as "aliasing," its result is to cause mathematically continuous geometries to have a "stair-stepped" or "jagged" appearance on the output display device, or to be omitted altogether, thereby significantly reducing the visual realism of the digital computer image.
The phenomenon of aliasing results from the conversion of the continuous lines and polygons in a geometric model into a raster, which consists of a finite array of picture elements, also known as "pixels" or "raster elements," for display on a cathode-ray tube or other output display device. For example, when the geometric model of an object is converted into a digital computer image, the object's geometry is mapped onto the finite array of pixels making up the image. Each pixel is assigned a color value based upon the color of the particular geometry defined in the geometric model, such as a polygon, for example, which occupies or "owns" a majority of that pixel. Therefore, if two adjoining polygons each occupy some portion of a pixel, the pixel will represent only that geometry occupying the majority of that pixel. Also, if neither polygon occupies a majority of the pixel, the pixel will not represent either of the geometries, even though some portion of the pixel is, in fact, occupied by the polygons. Therefore, due to the fact that each pixel is finite and discreet, the digital computer image of the geometric model may appear to have "stair-stepped" edges or a "hole."
Further, when the object to be displayed changes its position or orientation in the visual database with successive frame updates of the digital computer image, i.e. the object is dynamic, the stair-stepped effect also is dynamic, thereby causing the motion of the object to appear to be distorted by occurrences of "crawling edges," "hopscotching points," and other abrupt artificial effects unrelated to the true motion of the object. Aliasing has been found to be extremely undesirable in a device such as visual simulation system not only because it reduces the realism of the simulation, but also because it is a distraction to the system's operator and detracts from the simulation task at hand.
Some digital computer graphics processors provide a feature called "anti-aliasing" which significantly diminishes the unacceptable effects of aliasing. Anti-aliasing is accomplished by dividing each pixel in a raster with an array of as many as 16 sub-pixels. The sub-pixels are then processed in the image generator in the traditional manner, with the average color of all of the sub-pixels being used to determine the color value to be assigned to the pixel. This method "softens" and "masks" jagged edges, however, it presents certain drawbacks. One is that anti-aliased lines and edges have the appearance of being slightly out of focus. Another is that the number of processes required by the image generator to generate a given display image is multiplied by the number of sub-pixels in each pixel. This could potentially increase the number of processes required by the display processing unit by as many as 16 times. The combination of these two drawbacks require that much faster and, consequently, much more expensive digital computer graphic processing components be used so that higher resolution rasters may be utilized to maintain a sharp display image and so that the display image may be generated with enough speed to create the appearance of smooth motion for the dynamic objects in the visual scene.
However, in some specialized visual simulation system applications, complete anti-aliasing is not cost efficient, although some anti-aliasing capability is desired. These visual simulation system applications are characterized as having visual scenes with a large static portion, such as a background fixed with respect to a given viewpoint, and a small dynamic portion, such as one or more moving foreground objects set against the static background. Examples of applications like the one just described are illustrated by certain flight training visual simulation systems which involve object detection, recognition, and identification, such as in air defense or air traffic control tower trainers.